musepack: enable installation on linux

[reishoku]

Signed-off-by: KOSHIKAWA Kenichi <reishoku.misc @ pm.me>

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[cho-m]

See previous (unmerged) PR #92041 for changes needed to build.

Comment from @iMichka on maintenance status still holds.

[carlocab]

Suggested change

	if OS.mac?
	lib.install "libmpcdec/libmpcdec.dylib"
	else
	lib.install "libmpcdec/libmpcdec.so"
	end
	lib.install "libmpcdec/#{shared_library("libmpcdec")}"

Here's a more compact way of writing this.

It's not enough, unfortunately: #92041

[github-actions]

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